What is rollback segment

ROLLBACK INFORMATION NOTES: 
 
 
Segment Name - Name of the rollback segment.  
Owner - Owner of the rollback segment.  
Tablespace - Name of the tablespace containing the rollback segment.  
Segment ID - ID number of the rollback segment.  
File ID - ID number of the block containing the segment header.  
Block ID - Starting block number of the extent.  
Initial Extent - Initial extent size in bytes.  
Next Extent - Secondary extent size in bytes.  
Min Extents - Minimum number of extents.  
Max Extents - Maximum number of extents.  
PCT Increase - Percent increase for extent size.  
Status - ONLINE if the segment is online, or PENDING OFFLINE if the segment is going offline but some active (distributed) transactions are using the rollback segment. When the transaction(s) complete, the segment goes OFFLINE.  
Instance - Instance this rollback segment belongs to (Parallel Server), or NULL for a single-instance system .  
 
select SEGMENT_NAME, 
OWNER, 
TABLESPACE_NAME, 
SEGMENT_ID, 
FILE_ID, 
BLOCK_ID, 
INITIAL_EXTENT, 
NEXT_EXTENT, 
MIN_EXTENTS, 
MAX_EXTENTS, 
PCT_INCREASE, 
STATUS, 
INSTANCE_NUM 
from dba_rollback_segs 
order by SEGMENT_NAME 

Rollback segment is a database object,containing before image of data written to the database.It is used to..
:Undo changes when the transaction is rolled back.
:Recover the database in case of failures.
:Uncommitted changes made to the database,this will protected by it.